Trainer to the stars—she's worked with Carrie Underwood since 2007—Erin Oprea is in the business of getting people healthy and fit. Not only did she serve in the U.S. Marine Corp for nine years (!), but she's also a wife and mother of two, which makes her even more of an inspiration to whose who need a motivational boost to start working out and taking better care of themselves. Her new book, The 4 x 4 Diet, reveals her secrets for healthy eating and exercising on a tight schedule, which Carrie herself follows—she even calls Oprea her "fitness inspiration." We managed to get ahold of the fitness guru herself to find out her secrets for keeping some of Hollywood's biggest names in tip-top shape.

Set realistic goals. "You're going to be really motivated day one, so make sure you don't sprint and exhaust yourself physically and mentally all on the first day!" says Oprea. "Keeping a healthy lifestyle is a marathon, so stay consistent. A good weight loss is 1–2 pounds a week."

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Being "busy" shouldn't be an excuse.If you don't have time or money to make it to a gym, Oprea says there are plenty of ways to still get a workout in, which is a topic she focuses on in her book (seen in the picture below). "I'm all about taking advantage of your surroundings. Park benches, monkey bars, walls, ottomans, stairs and the ever-present jump rope can be done nearly anywhere. We are a very busy generation and need to get healthy on our time schedule, which means you fit it in when and where you can. I literally have jumped rope in my garage while it's raining, at all my kids' soccer events and even recently in an airport waiting for my plane. It may sound strange to many, but I don't usually have an hour to make it to a gym. Squats, lunges, dips and pushups can be done anywhere. Take full advantage of your body weight!"

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You can do a lot in four minutes."Tabatas are the best! This is the format that my book focuses on and you can incorporate all types of great, simple moves into a Tabata and get a ton of results out of it. A Tabata is a timed workout that has you exercising for 20 seconds, taking a 10-second break, then going right back into 20 more seconds of exercise. Repeat this for a total of eight rounds and voila, your four-minute workout is complete! It's always best to do a bunch back to back, but fit them in when you can. Do one right after you wake up to start your day, a couple during your lunch break and another when you're making dinner. Take squats and pushups for example: Do as many squats as you can with good form for 20 seconds, and during the 10-second break, get down and ready for pushups. Do as many pushups as you can for the next 20-second round. Go back and forth until all eight rounds are done. You'll feel the burn, I promise!"

What you eat is just as important. "There are key foods that are simple things anyone can incorporate into their nutrition program. Take for example my key for week one: Cut out starches in the evening. I don't really believe that things should be cut out altogether (if you want to, don't eat any more cookies—I'm OK with this). Carbohydrates are your energy source and you really need to eat healthy versions like oatmeal or sweet potatoes. These are great! Just eat them earlier in the day so you use them as energy. No need to have all that energy right before bed! Stick to great vegetables in the evening like broccoli or spinach so you still get carbs, just not starchy ones like pasta."

It's not about perfection. "I want you to remember that the whole world is your gym. Fit in fitness where and when you can and it doesn't need to be perfect! Think about it like this: You buy a case for your phone and you're going to replace that in a year or two. Start taking care of your body now because you don't get to sign another contract!"